Response of Earthworm Eisenia fetida to the Stresses Induced by Salinity and Lead Pollution in a Soil Amended with Cow Manure

Salinity and pollution are two environmental stresses that individually influence the population, growth and activity of earthworms as soil bioengineers. It is well-known that the population and activity of these organisms are mostly reduced or even their activity and growth can be stopped in polluted and saline soils. The individual effects of these abiotic stresses on earthworms, however, depend on the level of salinity, pollution and organic matter. Nonetheless, the joint or combined effect of these stresses on earthworms, especially in arid and semi-arid areas, is poorly known. Because of the importance of earthworms in soil ecosystem, the study of salinity and pollution interactions on earthworm population and activity to reduce their detrimental effects using organic materials is essential. The aim of this study was to examine how salinity and lead (Pb) stresses simultaneously affect the earthworms in soil ecosystem.
In this research, the interaction effect of salinity stress using sodium chloride (NaCl) and Pb stress using lead nitrate (PbNO3) on the population, weight and activity of the earthworm Eisenia fetida was studied under greenhouse conditions. This factorial experiment was carried out using 3 factors, including Pb pollution (control and 30 mg kg-1 Pb), salinity (control, 4 and 8 dS m-1) and cow manure (control and 4% by weight) arranged in a completely randomized design with four replicates. The experiment lasted 13 weeks and earthworm’s population and activity including the number of adult worms, total earthworms, wet and dry weights, and wet and dry weights of casts produced by earthworm were measured at the end of the experiment. Concentration of DTPA (di-ethylene-triamine-pentaacetic acid) extractable Pb was also determined to assess how salinity influences the accessibility of this metal in the soil. The Fisher’s least significant difference test was used to determine the significance of any difference between the means values at 5% level with the STATISTICA 8 software. The Bliss Independence Model was used to determine the type of interaction between salinity and Pb pollution for each manure treatment.
The current results showed that increasing salinity level enhanced the accessibility of Pb and subsequently its toxicity for earthworms. In contrast, addition of cow manure reduced the accessibility of Pb by 22-50% at all salinity levels. Earthworm population, wet and dry body weights, and wet and dry weights of casts produced by worms were all significantly (p